In a GSM system the transmission of speech over the radio interface can be governed by discontinuous transmission/reception. In this mode the radio modem transmitter is turned off during the silence period in a conversation except for the occasional transmission of the Silence Descriptor (SID) frame. When speech returns, speech frames are transmitted continuously. Each frame encodes a 20 ms slot.
In the transmitter radio subsystem the presence or absence of speech is indicated by a speech flag (SP) which is provided by the speech encoder algorithm. When SP=1, speech frames are sent and when SP=0 SID frames are synthesised and transmitted every 24 frames.
At the receiver, the channel decoder is able to differentiate between a) speech frames b) SID frames and c) bad frames. This differentiation is achieved by monitoring the frame type indicator. The absence of frames (silence) can be detected by the equaliser which will not receive a training sequence in that state.
Providing for group calls using a GSM based system in a spectrally efficient manner involves the utilisation of the same uplink and downlink traffic channel (TCH) by all the mobiles in the group.
Considering the fact that only one mobile is permitted to talk (and therefore broadcast) at any one time we have a push-to-talk (PTT) scenario. When one mobile is broadcasting all the others should be inhibited from transmitting. In a simple system the users are relied upon to refrain from speaking when the channel is free.
In known PMR systems, it has been proposed that the existence of carrier on the channel inhibits the PTT function in a mobile radio, thereby giving a orderly contention for access to the channel. This is a crude arrangement which can result in one user "hogging" the channel, deliberately or inadvertently. Inadvertent hogging of the channel can take place when a user forgets to release the PTT button.
There is a need for a more flexible manner of group contention.